Sputter depositions are reproducible and are also simple for process automation. Rotary sputtering targets have been shown to improve the sputtering process. By rotating the sputtering material, the target material is sputtered in a more continuous uniform fashion than when compared to planar targets. Rotary sputtering targets are typically formed in a cylindrical shape.
Of particular concern with rotary sputtering targets, are with the use of soft or malleable material having a high weight to strength ratio. Such materials include sliver and gold. As more material is sputtered away, the rotary sputtering target becomes thinner and thus more likely to flex or break during rotation. To alleviate this problem typically the rotary sputtering target is supported by a backing tube.
The backing tube is constructed of a material that can withstand the sputtering process while retaining its shape. The backing tube supports the rotary sputtering target thereby reducing excess flexing of the target material.
Another advantage of using a backing tube is that cooling fluid can be passed through the backing tube to cool the target during the sputtering process. For cooling to be effective it is important that there is sufficient thermal contact between the cooling fluid and the rotary sputtering target. Current technology for the assembly of rotary sputtering targets attach the backing tube in such a manner as to prevent any gap between the inners surface of the rotary sputtering target and the outer surface of the backing tube to ensure sufficient thermal contact between the backing tube and the target material.
Generally, backing tubes are made from a material having low thermal conductive properties. This is especially important for rotary target material having a low melting point. Using a backing tube having a high thermal conductivity could result in thermal gain which would lead to an incipient melting situation. This would cause a short in the sputtering process and dangerous arching in the system.
A further advantage of using backing tubes is that the backing tube is formed from a material having a lower cost than the target material. The backing tube allows for more of the target material to be sputtered without deformation and therefore, produces higher yields when compared to systems lacking a backing tube.
There exist various techniques for attaching the rotary sputtering target to the backing tube. One such technique is to cast the rotary sputtering target onto the backing tube. This in essence creates a complete structure that is very rigid. However, casting is only feasible when using a castable rotary sputtering target material. Casting directly on the backing tube has its drawbacks. For example, casting often results in variable grain size and has an inherent porosity that is created from the volume changes of the liquid to solid transition. These variables and defects are detrimental to target system.
Another method for attachment of the rotary sputtering target to the backing tube is by use of a mechanical, such as by the use of sleeves. However, this methodology creates problems in the sputtering process. During the sputtering process heat is generated at the outer surface of the target material due to the plasma created at the surface. Heat is then transferred into the bulk of the target through to the inner surfaces of the target. To cool the inner surfaces, a coolant typically flows through the backing tube. However, due to thermal expansion of the sleeves relative to that of the target material and the backing tube, the inner surface of the rotary sputtering target may lose contact with the backing tube, and thus limit the cooling effect achieved by being in contact with the backing tube. Once contact with the backing tube is broken thermal expansion increases, exacerbating the problem even further.
Another technique for attaching the rotary sputtering target to the backing tube is by use of a bonding material. An adhesive or metal alloy is placed between the inner surface of the rotary sputtering target and the outer surface of the backing tube. This creates a strong bond between the two surfaces and assists in cooling transfer. However, care must be taken when flowing the adhesive between the rotary sputtering target and the backing tube to ensure adequate bonding strength. Reuse of the backing tube is a laborious process, as it is difficult to remove the remaining target material from a sufficiently secure bond.
One method to improve cooling transfer to the target material is to modify the backing tube. One method is to remove material creating depressions on the inner surface of the backing tube such as discussed in WO 2009/036910 to Preissler et al.
Another method to improve cooling transfer is to perforate the backing tube. Such perforations are discussed in EP1813695 to De Bosscher, the contents of which are incorporated herein by reference. The perforations in the backing tube allow for direct contact of the cooling fluid with the target material. If direct contact of the cooling fluid is not desired, a membrane that is leak proof and thermally conductive is used between the target material and the backing tube. To attach the rotary sputtering target to the backing tube, sleeves having vacuum to water seals are used to both support the target material and prevent leakage of the cooling fluid. Alternatively, the rotary sputtering target is milled while the backing tube is contained completely within the sleeve.
Having a low cost and high weight to strength ratio backing tube is desired.
Additionally, being able to reliably bond rotary sputtering targets to backing tubes is desired.
Moreover, being able to attaching a rotary sputtering target to a backing tube for easy removal and reuse of the backing tube is desired.
Also, having a jig for repeatable bonding a rotary sputtering target to a backing tube is desired.
In addition, directly contacting cooling fluid with a rotary sputtering target to improve cooling in particular circumstances is desired.